Liquid crystalline transfer sheet and process of producing the same

ABSTRACT

A liquid crystalline transfer sheet  10  includes a liquid crystal layer  12  formed on the surface of a substrate  14 , where the surface hardness of the liquid crystal layer  12  is higher on the releasing surface  12 A side, which is on the substrate  14  side, than on the adhering surface  12 B side, which is on the receiving object  16  side. Therefore, the adhesion between the adhering surface  12 B of the liquid crystal layer  12  and the receiving object  16  becomes stronger than that between the releasing surface  12 A of the liquid crystal layer  12  and the substrate  14 . It is thus possible to securely transfer the liquid crystal layer  12  to the receiving object  16  without leaving a part of the liquid crystal layer  12  on the substrate  14.

TECHNICAL FIELD

The present invention relates to a liquid crystalline transfer sheet fortransferring, to a receiving object, a liquid crystal layer havingoptical activities such as light-polarizing activity. More particularly,the present invention relates to a liquid crystalline transfer sheet fortransferring a liquid crystal layer formed on the surface of a substrateto a receiving object by separating the liquid crystal layer from thesubstrate, and to a process of producing such a liquid crystallinetransfer sheet.

BACKGROUND ART

In the liquid crystalline transfer sheet as described above, a releaselayer or easily separable adhesive layer has conventionally beenprovided between the liquid crystal layer and the substrate so that theliquid crystal layer can easily be separated from the substrate, therebymaking it possible to successfully transfer the liquid crystal layer toa receiving object.

To transfer the liquid crystal layer to a receiving object, it isnecessary to stick the liquid crystal layer to the receiving object.Having been commonly employed for this purpose is such a conventionalmeans that an adhesive layer is provided between the liquid crystallayer and a receiving object to which the liquid crystal layer istransferred. Specifically, an adhesive layer is provided in advanceeither on the transfer-side surface of the liquid crystal layer (surfaceto be adhered to a receiving object) or on a receiving object.Alternatively, an adhesive layer is formed on the transfer-side surfaceof the liquid crystal layer or on a receiving object in the process oftransferring the liquid crystal layer to the receiving object.

Another means for sticking the liquid crystal layer to a receivingobject is thermocompression bonding.

The above-described liquid crystalline transfer sheet containing arelease layer or easily separable adhesive layer provided between theliquid crystal layer and the substrate is to have an increased number oflayers, so that it has complicated constitution of lamination. Moreover,the substances constituting the release layer or easily separableadhesive layer can mingle with the liquid crystal layer or can partiallystick to the liquid crystal layer when the liquid crystal layer isseparated; the liquid crystal layer that has been transferred to areceiving object can thus show lowered optical properties.

Of the aforementioned liquid crystalline transfer sheets, a liquidcrystalline transfer sheet of the type which the liquid crystal layer isadhered to a receiving object through an adhesive layer isdisadvantageous in that the liquid crystal layer transferred to thereceiving object shows lowered optical properties because of theadhesive layer provided. One cause of this lowering of the opticalproperties is interfacial reflection that occurs at the interface of theadhesive layer and the liquid crystal layer as well as at the interfaceof the adhesive layer and the receiving object. Another cause is asfollows: the adhesive layer changes its shape because of its fluiditywhen transferred to a highly even receiving object, so that thethickness of the adhesive layer becomes non-uniform; as a result, theliquid crystal layer adhered to the receiving object through such anadhesive layer is to have decreased evenness.

Further, it is not easy to make an adhesive layer thin (1 μm or less),so that it inevitably has a thickness to some extent. Many adhesivelayers are therefore colored, or cause separation, or yellow, whenheated as disclosed in Japanese Laid-Open Patent Publications No.313729/1996, No. 29325/1999, No. 75924/1996, No. 151877/1999, etc. Inparticular, adhesive layers made from acrylic resins, as disclosed inJapanese Laid-Open Patent Publication No. 28827/2000, are known toyellow at high temperatures of more than 200° C.

Of the above-described liquid crystalline transfer sheets, a liquidcrystalline transfer sheet of the type which the liquid crystal layer isadhered to a receiving object via thermocompression bonding is alsodisadvantageous in that it is not easy to peel the substrate from theliquid crystal layer after the liquid crystal layer formed on thesubstrate has been adhered to a receiving object via thermocompressionbonding, if the release layer, easily separable adhesive layer oradhesive layer as described above is not provided.

The above-described phenomenon depends on materials for the substrateand the receiving object, and occurs when the adhesion between theliquid crystal layer to be transferred and the substrate is strongerthan the adhesion between the liquid crystal layer and the receivingobject. In such a case, it is not easy to peel the substrate from theliquid crystal layer after the liquid crystal layer has been adhered tothe receiving object. In particular, if the liquid crystal layer isextremely thin, it can be broken when the substrate is tried to peelfrom the liquid crystal layer. In addition, as disclosed in JapaneseLaid-Open Patent Publication No. 311710/1999, for example, there is sucha case where a cholesteric liquid crystal layer is adhered to anothercholesteric liquid crystal layer via thermocompression bonding. Even inthis case, if the cholesteric liquid crystal layers are thin or theadhesion between the cholesteric liquid crystal layer and the substrateis stronger than that between the cholesteric liquid crystal layers, itis difficult to transfer the cholesteric liquid crystal layers to areceiving object, and the cholesteric liquid crystal layers can bedamaged (broken).

DISCLOSURE OF THE INVENTION

The present invention was accomplished in the light of theabove-described drawbacks in the related art. An object of the presentinvention is to provide a liquid crystalline transfer sheet comprising aliquid crystal layer adapted to securely and easily be transferred,without being damaged, to a receiving object, even if a release layer,easily separable adhesive layer or adhesive layer is not provided, andto provide a process of producing such a liquid crystalline transfersheet.

A first aspect of the present invention is a liquid crystalline transfersheet comprising: a substrate; and a liquid crystal layer formed on thesurface of the substrate, the liquid crystal layer being able to beadhered to a receiving object with its adhering surface, the surface onthe side opposite to the substrate, and being able to be separated fromthe substrate at its releasing surface, the surface on the substrateside, wherein the surface hardness of the liquid crystal layer is loweron the adhering surface side than on the releasing surface side.

In the liquid crystalline transfer sheet according to the first aspectof the present invention, it is preferable that the liquid crystal layerbe made from polymerizable liquid crystalline molecules and that therate of residual double bonds of the liquid crystalline molecules in thevicinity of the adhering surface of the liquid crystal layer be higherthan that of the liquid crystalline molecules in the vicinity of thereleasing surface of the liquid crystal layer. It is also preferablethat the rate of residual double bonds of the liquid crystallinemolecules in the vicinity of the releasing surface of the liquid crystallayer is 60% or less of that of the liquid crystalline molecules in thevicinity of the adhering surface of the liquid crystal layer.

Further, in the liquid crystalline transfer sheet according to the firstaspect of the present invention, it is preferable that the liquidcrystal layer be composed of a plurality of thin liquid crystal layerssuccessively laminated. It is also preferable that each thin liquidcrystal layer be made from polymerizable liquid crystalline moleculesand that the rate of residual double bonds of the liquid crystallinemolecules in the thin liquid crystal layer in the vicinity of theadhering surface be higher than that of the liquid crystalline moleculesin the thin liquid crystal layer in the vicinity of the releasingsurface. It is also preferable that the rate of residual double bonds ofthe liquid crystalline molecules in the thin liquid crystal layer thatforms the releasing surface is 60% or less of that of the liquidcrystalline molecules in the thin liquid crystal layer that forms theadhering surface.

Furthermore, in the liquid crystalline transfer sheet according to thefirst aspect of the present invention, it is preferable that the liquidcrystal layer made from the liquid crystalline molecules be cholesteric.It is also preferable that the substrate be an oriented film. It is alsopreferable that an alignment layer be formed on one surface of thesubstrate, the surface being brought into contact with the releasingsurface of the liquid crystal layer.

The second aspect of present invention is a process of producing aliquid crystalline transfer sheet comprising a substrate and a liquidcrystal layer formed on the surface of the substrate, the liquid crystallayer being able to be adhered to a receiving object with its adheringsurface, the surface on the side opposite to the substrate, and beingable to be separated from the substrate at its releasing surface, thesurface on the substrate side. The process comprises the steps of:forming, on a substrate, a liquid crystal layer by using liquidcrystalline molecules polymerizable by application of radiation; andcuring the liquid crystal layer by application of radiation in anatmosphere of air so that the surface hardness of the liquid crystallayer is lower on the adhering surface side than on the releasingsurface side.

In the process of producing a liquid crystal line transfer sheetaccording to the second aspect of the present invention, it ispreferable that the atmosphere of air be an atmosphere with an oxygenconcentration of 0.5% or more.

A third aspect of the present invention is a process of producing aliquid crystalline transfer sheet comprising a substrate and a liquidcrystal layer composed of a plurality of thin liquid crystal layers,formed on the surface of the substrate, the liquid crystal layer beingable to be adhered to a receiving object with its adhering surface, thesurface on the side opposite to the substrate, and being able to beseparated from the substrate at its releasing surface, the surface onthe substrate side. The process comprises the steps of: forming, on asubstrate, a thin liquid crystal layer by using liquid crystallinemolecules polymerizable by application of radiation; curing the thinliquid crystal layer by application of radiation; forming an additionalthin liquid crystal layer on the cured thin liquid crystal layer byusing liquid crystalline molecules polymerizable by application ofradiation; and curing the additional thin liquid crystal layer byapplication of radiation, wherein the step of forming the additionalthin liquid crystal layer and the step of curing the additional thinliquid crystal layer are repeated one or more times to successivelylaminate the desired number of thin liquid crystal layers, and thedegree of cure of each thin liquid crystal layer is controlled so thatthe hardness of the thin liquid crystal layer in the vicinity of theadhering surface is lower than that of the thin liquid crystal layer inthe vicinity of the releasing surface.

In the process of producing a liquid crystalline transfer sheetaccording to the third aspect of the present invention, it is preferableto control the degree of cure of each thin liquid crystal layer byadjusting the oxygen concentration of the atmosphere in which radiationis applied to the thin liquid crystal layer. It is also preferable tocontrol the degree of cure of each thin liquid crystal layer byadjusting the amount of radiation to be applied to the thin liquidcrystal layer.

A fourth aspect of the present invention is a process of producing aliquid crystalline transfer sheet comprising a substrate and a liquidcrystal layer composed of a plurality of thin liquid crystal layers,formed on the surface of the substrate, the liquid crystal layer beingable to be adhered to a receiving object with its adhering surface, thesurface on the side opposite to the substrate, and being able to beseparated from the substrate at its releasing surface, the surface onthe substrate side. The process comprises the steps of: laminating, on asubstrate, the desired number of thin liquid crystal layers by usingliquid crystalline molecules polymerizable by application of ultravioletlight; and curing the laminated thin liquid crystal layers byapplication of ultraviolet light, wherein the degree of cure of eachthin liquid crystal layer Is controlled by adjusting the amount of aphotopolymerization initiator to be added to the thin liquid crystallayer so that the hardness of the thin liquid crystal layer in thevicinity of the adhering surface is lower than that of the thin liquidcrystal layer in the vicinity of the releasing surface.

According to the present invention, the surface hardness of the liquidcrystal layer on its releasing surface side, which is on the substrateside, and that of the liquid crystal layer on its adhering surface side,which is on the receiving object side, are respectively adjusted, sothat the adhesion between the adhering surface of the liquid crystallayer and a receiving object will be stronger than that between thereleasing surface of the liquid crystal layer and the substrate. It istherefore possible to securely stick the liquid crystal layer to thereceiving object and, at the same time, to easily peel the substratefrom the liquid crystal layer. It is thus possible to securely andeasily transfer the liquid crystal layer to a receiving object withoutdamaging the liquid crystal layer or partially leaving the liquidcrystalline component of the liquid crystal layer on the substrate, evenif a release layer, easily separable adhesive layer or adhesive layer isnot provided.

Further, according to the present invention, since it is not necessaryto provide a release layer or easily separable adhesive layer betweenthe liquid crystal layer and the substrate of the liquid crystallinetransfer sheet, the liquid crystalline transfer sheet of the inventionis simple in the constitution of lamination and has no chance thatsubstances constituting a release layer or easily separable adhesivelayer mingle with the liquid crystal layer or partially remain on theliquid crystal layer in the course of separation.

Furthermore, according to the present invention, there is no need toprovide an adhesive layer between the liquid crystal layer of the liquidcrystalline transfer sheet and a receiving object, so that only adecreased number of interfaces are to exist. Therefore, the liquidcrystal layer does not undergo lowering of the optical properties thatis caused by interfacial reflection. Moreover, since the liquid crystallayer is directly adhered to the receiving object, if the evenness ofthe receiving object is high, this evenness can be maintained, andlowering of the optical properties of the liquid crystal layer can thusbe prevented. In addition, the liquid crystal layer is free from changein color tone, separation, and yellowing upon heating, which are broughtabout by the adhesive layer.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings,

FIG. 1 is an enlarged diagrammatical cross-sectional view showing aliquid crystalline transfer sheet according to the first embodiment ofthe present invention;

FIG. 2 is a diagrammatical cross-sectional view illustrating a processof producing the liquid crystalline transfer sheet shown in FIG. 1;

FIG. 3 is an enlarged diagrammatical cross-sectional view showing aliquid crystalline transfer sheet according to the second embodiment ofthe present invention;

FIG. 4 is a diagrammatical cross-sectional view illustrating a processof producing the liquid crystalline transfer sheet shown in FIG. 3;

FIG. 5 is a diagrammatical cross-sectional view illustrating anotherprocess of producing the liquid crystalline transfer sheet shown in FIG.3;

FIG. 6 is a graph showing the relationship between logarithmic decrementand temperature on the adhering surface of a liquid crystalline transfersheet according to one example of the present invention; and

FIG. 7 is a graph showing the relationship between logarithmic decrementand temperature on the adhering surface of a comparative liquidcrystalline transfer sheet.

BEST MODE FOR CARRYING OUT THE INVENTION

By referring to the accompanying drawings, embodiments of the presentinvention will be described hereinafter.

First Embodiment

The first embodiment of the present invention will be firstly describedby referring to FIGS. 1 and 2.

As shown in FIG. 1, a liquid crystalline transfer sheet 10 according tothe first embodiment of the present invention includes a liquid crystallayer 12 formed on the surface of a substrate 14 made of an oriented PET(polyethylene terephthalate) film, for example. The liquid crystal layer12 is made so that it can be adhered to a receiving object 16 with itsadhering surface 12B, the surface on the side opposite to the substrate14. The liquid crystal layer 12 is also made so that it can be separatedfrom the substrate 14 at its releasing surface 12A, the surface on thesubstrate 14 side. The surface hardness of the liquid crystal layer 12is made higher on the releasing surface 12A side, on which the liquidcrystal layer 12 is separated from the substrate 14, than on theadhering surface 12B side, on which the liquid crystal layer 12 isadhered to the receiving object 16.

The liquid crystal layer 12 is formed by the use of a photopolymerizableliquid crystal (e.g., cholesteric liquid crystal), which is orientedwhen brought into contact with the substrate 14 made of an oriented PETfilm, and its surface hardness is made different between the releasingsurface 12A side and the adhering surface 12B side as described above bycontrolling the curing conditions, as will be described later. To formthe liquid crystal layer 12, not only liquid crystalline molecules(liquid crystalline monomer or oligomer) polymerizable by theapplication of ultraviolet light or the like but also a polymeric liquidcrystal can be used, as will be described later.

The term “liquid crystal layer” herein refers to a film whose certainpart has the properties (especially, optical properties) of liquidcrystals and does not mean the state of liquid crystalline phase in aphysical sense. For example, the liquid crystal layer herein includeseven a non-fluid film as long as it has been solidified with themolecular orientation of liquid crystal phase (e.g., cholesteric liquidcrystal phase) maintained.

Next, a process of producing the liquid crystalline transfer sheet 10shown in FIG. 1 (method for making the surface hardness of the liquidcrystal layer 12 different between the releasing surface 12A side andthe adhering surface 12B side) will be explained by referring to FIG. 2.The explanation will be given by taking as an example a case where acholesteric liquid crystalline monomer polymerizable by the applicationof ultraviolet light is used to form the liquid crystal layer 12.

A cholesteric liquid crystalline monomer solution containing aphotopolymerization initiator is firstly prepared. This solution isapplied, as indicated by reference numeral 11 in FIG. 2(A), to thesurface of the substrate 14 made of an oriented PET film and is thendried to remove the solvent, thereby forming an uncured liquid crystallayer 11A as shown in FIG. 2(B).

Next, as shown in FIG. 2(B), ultraviolet light is applied to thisuncured liquid crystal layer 11A and the substrate 14 from the liquidcrystal layer 11A side in an atmosphere of air (oxygen concentration:approximately 20%), thereby curing the liquid crystal layer 11A. Aliquid crystalline transfer sheet 10 including a cured liquid crystallayer 12 can thus be obtained as shown in FIG. 2(C).

The adhering surface 12B of the liquid crystal layer 12 is exposed tothe air, so that cleavage of C═C bonds in the liquid crystallinemolecules (e.g., double bond between carbon atoms in acrylic groupcontained in the cholesteric liquid crystalline molecule, etc.) aresuppressed. On the other hand, the releasing surface 12A of the liquidcrystal layer 12 is not in contact with oxygen, so that curing proceedsmore greatly on the releasing surface 12A side. The surface hardness ofthe liquid crystal layer 12 thus becomes lower on the adhering surface12B side than on the releasing surface 12A side.

For this reason, when the liquid crystal layer 12 is transferred to thereceiving object 16, as shown in FIG. 1, the adhesion between theadhering surface 12B of the liquid crystal layer 12 and the receivingobject 16 becomes stronger than that between the releasing surface 12Aof the liquid crystal layer 12 and the substrate 14. The substrate 14can thus be easily peeled from the liquid crystal layer 12 with theliquid crystal layer 12 adhered to the receiving object 16.

The surface hardness (degree of cure at the surface) of the liquidcrystal layer 12 can be determined by the rate of residual double bondsof the liquid crystalline molecules in the vicinity of the releasingsurface 12A and that of the liquid crystalline molecules in the vicinityof the adhering surface 12B. (The “liquid crystalline molecules” hereininclude monomers, oligomers, polymers and every other liquid crystallinecompound having reactive C═C bonds.)

The “rate of residual double bonds” as used herein is defined asfollows:

Rate of residual double bonds=[Spectral band intensity of absorption (inthe vicinity of 810 cm⁻¹) characteristic of C═C bonds in liquidcrystalline molecules]÷[Spectral band intensity of absorption (in thevicinity of 1500 cm⁻¹) characteristic of aromatic rings in liquidcrystalline molecules].

The above-defined rate of residual double bonds directly indicates therate of unreacted double bonds (C═C bonds) remaining afterpolymerization reaction, and [(1−(rate of residual double bonds)]indicates the degree of conversion (degree of polymerization).

As mentioned above, the surface hardness of the liquid crystal layer 12is made lower on the adhering surface 12B side than on the releasingsurface 12A side. This means that the rate of residual double bonds ofthe liquid crystalline molecules in the vicinity of the adhering surface12B of the liquid crystal layer 12 is made higher than that of theliquid crystalline molecules in the vicinity of the releasing surface12A of the liquid crystal layer 12. Preferably, the surface hardness ofthe liquid crystal layer 12 on the adhering surface 12B side and that ofthe liquid crystal layer 12 on the releasing surface 12A side are madeso that the rate of residual double bonds on the releasing surface 12Aside will be 60% or less of that on the adhering surface 12B side.

In the case where the liquid crystal layer 11A is cured by theapplication of ultraviolet light, the oxygen concentration of theatmosphere of air under which the curing is conducted is preferably 0.5%or more. This is because, if the oxygen concentration is too low, nodifference in degree of conversion is caused between the releasingsurface 12A side and adhering surface 12B side of the liquid crystallayer 12.

Radiation such as an electron beam may be applied instead of ultravioletlight. In this case, a photopolymerization initiator may not be added tothe cholesteric liquid crystalline monomer solution that is applied tothe surface of the substrate 14.

According to the liquid crystalline transfer sheet 10 of the firstembodiment of the present invention, the surface hardness of the liquidcrystal layer 12 on the releasing surface 12A side, which is on thesubstrate 14 side, and that of the liquid crystal layer 12 on theadhering surface 12B side, which is on the receiving object 16 side, areadjusted so that the adhesion between the adhering surface 12B of theliquid crystal layer 12 and the receiving object 16 will be strongerthan that between the releasing surface 12A of the liquid crystal layer12 and the substrate 14. Therefore, it is possible to securely stick theliquid crystal layer 12 to the receiving object 16, and, at the sametime, to easily peel the substrate 14 from the liquid crystal layer 12.It is thus possible to securely and easily transfer the liquid crystallayer 12 to the receiving object 16 without damaging the liquid crystallayer 12 or partly leaving the liquid crystalline component of theliquid crystal layer 12 on the substrate 14, even if a release layer,easily separable adhesive layer or adhesive layer is not employed.

Second Embodiment

A liquid crystalline transfer sheet according to the second embodimentof the present invention will be described by referring to FIGS. 3 and4. The second embodiment of the present invention is basically the sameas the aforementioned first embodiment except that the liquid crystallayer in the second embodiment is composed of a plurality of thin liquidcrystal layers. It is noted that like reference characters designatelike or corresponding parts in the accompanying drawings and that thoseparts that have been explained in the description of the firstembodiment will not be explained anymore in detail in the description ofthis second embodiment.

As shown in FIG. 3, a liquid crystalline transfer sheet 20 according tothe second embodiment of the present invention includes a liquid crystallayer 22 composed of two (or three or more) thin liquid crystal layers24A and 24B successively laminated, formed on the surface of a substrate14 made of an oriented PET (polyethylene terephthalate) film, forexample. This liquid crystal layer 22 is made so that it can be adheredto the substrate 14 with its adhering surface 22B, the surface on theside opposite to the substrate 14, and that it can be separated from thesubstrate 14 at its releasing surface 22A, the surface on the substrate14 side. The hardness of the thin liquid crystal layer 24A that formsthe releasing surface 22A at which the liquid crystal layer 22 isseparated from the substrate 14 is made higher than that of the thinliquid crystal layer 24B that forms the adhering surface 22B with whichthe liquid crystal layer 22 is adhered to a receiving object 16. In thecase where the thin liquid crystal layers 24A and 24B are made frompolymerizable liquid crystalline molecules, the proportion of thehardness of the thin liquid crystal layer 24A to that of the thin liquidcrystal layer 24B is determined by the proportion of the rate ofresidual double bonds of the liquid crystalline molecules in the thinliquid crystal layer 24A to that of the liquid crystalline molecules inthe thin liquid crystal layer 24B like in the case of the liquid crystallayer 12 of the liquid crystalline transfer sheet 10 according to theabove-described first embodiment. For instance, the rate of residualdouble bonds of the liquid crystalline molecules in the thin liquidcrystal layer 24A provided on the releasing surface 22A side may be made60% or less of that of the liquid crystalline molecules in the thinliquid crystal layer 24B provided on the adhering surface 22B side.

Next, by referring to FIG. 4, a process of producing the liquidcrystalline transfer sheet 20 shown in FIG. 3 will be explained. Theexplanation is herein given by taking as an example a case where acholesteric liquid crystalline monomer polymerizable by the applicationof ultraviolet light is used to form the liquid crystal layer 22.

A cholesteric liquid crystalline monomer solution containing aphotopolymerization initiator is firstly prepared. This solution isapplied, as indicated by reference numeral 21A in FIG. 4(A), to thesurface of a substrate 14 made of the same oriented PET film as thatused in the above described first embodiment and is then dried to removethe solvent, thereby forming an uncured thin liquid crystal layer 23A asshown in FIG. 4(B).

Next, as shown in FIG. 4(B), ultraviolet light is applied to thisuncured thin liquid crystal layer 23A in an atmosphere of nitrogen(oxygen concentration: 0.5% or less) to cure this layer. A cured thinliquid crystal layer 24A is thus formed as shown in FIG. 4(C).

Thereafter, the same cholesteric liquid crystalline monomer solution asthe above-described one is applied, as indicated by reference numeral21B in FIG. 4(D), to the surface of the cured thin liquid crystal layer24A in the manner as described above, thereby forming an uncured thinliquid crystal layer 23B as shown in FIG. 4(E).

As shown in FIG. 4(E), ultraviolet light is applied to this uncured thinliquid crystal layer 23B in an atmosphere of air (oxygen concentration:0.5% or more) to cure this layer, thereby obtaining a cured thin liquidcrystal layer 24B as shown in FIG. 4(F). A liquid crystalline transfersheet 20 including a liquid crystal layer 22 composed of the cured thinliquid crystal layers 24A and 24B laminated is thus finally obtained.

To form a liquid crystal layer composed of three or more thin liquidcrystal layers, the above-described steps (FIGS. 4(D) to 4(F)) arerepeated to successively laminate thin liquid crystal layers in a numberdesired.

The thin liquid crystal layer 24A is oriented by the aligning action ofthe substrate 14 made of an oriented PET film. On the other hand, thethin liquid crystal layer 24B is oriented when it is brought into directcontact with the thin liquid crystal layer 24A that has been cured withthe state of orientation maintained.

The thin liquid crystal layer 24A is cured in an atmosphere of nitrogen,while the thin liquid crystal layer 24B is cured in an atmosphere ofair. Therefore, the degree of cure of the thin liquid crystal layer 24Bbecomes lower than that of the thin liquid crystal layer 24A. In otherwords, the hardness of the thin liquid crystal layer 24A, which formsthe releasing surface 22A of the liquid crystal layer 22 at which theliquid crystal layer 22 is separated from the substrate 14, becomeshigher than that of the thin liquid crystal layer 24B, which forms theadhering surface 22B of the liquid crystal layer 22 with which theliquid crystal layer 22 is adhered to a receiving object 16.Consequently, the adhesion between the thin liquid crystal layer 24B andthe receiving object 16 becomes stronger than that between the thinliquid crystal layer 24A and the substrate 14.

It is possible to control the degree of cure of each thin liquid crystallayer 24A or 24B by adjusting the oxygen concentration of the atmospherein which ultraviolet light is applied to the thin liquid crystal layer24A or 24B. Alternatively, the degree of cure of each thin liquidcrystal layer 24A or 24B can also be controlled by adjusting the amountof the photopolymerization initiator to be added, the amount ofultraviolet light to be applied, or a combination of these two.

In the case where the degree of cure of each thin liquid crystal layer24A or 24B is controlled by adjusting the amount of thephotopolymerization initiator to be added, after successively laminatinga plurality of uncured thin liquid crystal layers 23A and 23B (FIGS.5(A) and 5(B)), ultraviolet light is applied only once to these thinliquid crystal layers 23A and 23B laminated, as shown in FIG. 5. Aliquid crystalline transfer sheet 20 including a liquid crystal layer 22composed of the cured thin liquid crystal layers 24A and 24B laminatedcan thus be obtained.

In the case where the degree of cure of each thin liquid crystal layer24A or 24B is controlled by adjusting the oxygen concentration of theatmosphere in which ultraviolet light is applied, or by adjusting theamount of ultraviolet light to be applied, it is possible to useradiation such as an electron beam instead of ultraviolet light. In thiscase, a photopolymerization initiator may not be added to thecholesteric liquid crystalline monomer solution that is applied to thesurface of the substrate 14.

According to the liquid crystalline transfer sheet 20 of the secondembodiment of the present invention, the hardness of the thin liquidcrystal layer 24A, which forms the releasing surface 22A of the liquidcrystal layer 22 on the substrate 14 side, and the hardness of the thinliquid crystal layer 24B, which forms the adhering surface 22B of theliquid crystal layer 22 on the receiving object 16 side, are adjusted sothat the adhesion between the adhering surface 22B of the liquid crystallayer 22 and the receiving object 16 will be stronger than that betweenthe releasing surface 22A of the liquid crystal layer 22 and thesubstrate 14. It is therefore possible to securely stick, to thereceiving object 16, the liquid crystal layer 22 composed of theplurality of thin liquid crystal layers 24A and 24B and, at the sametime, to easily peel the substrate 14 from the liquid crystal layer 22.It is thus possible to securely and easily transfer the liquid crystallayer 22 to the receiving object 16 without damaging the liquid crystallayer 22, even if a release layer, easily separable adhesive layer oradhesive layer is not employed.

In the aforementioned first and second embodiments of the invention,ultraviolet light is applied from the liquid crystal layer side to theliquid crystal layer formed on the substrate 14. However, the presentinvention is not limited to this, and ultraviolet light may also beapplied to the liquid crystal layer through the substrate 14 if thesubstrate 14 transmits or scarcely absorbs ultraviolet light. In thiscase, by controlling the thickness of the liquid crystal layer, theamount of the photopolymerization initiator to be added, the amount ofultraviolet light to be absorbed by the liquid crystal itself, or, ifnecessary, the amount of an ultraviolet light absorber to be added, theamount of ultraviolet light to be applied to the releasing surface ofthe liquid crystal layer may be made different from that of ultravioletlight to be applied to the adhering surface of the liquid crystal layerso that the ratio of the former to the latter will be approximately10:6.

Further, in the above-described first and second embodiments,cholesteric liquid crystals having cholesteric phase, composed of liquidcrystalline molecules are used to form the liquid crystal layers 12 and22. The present invention, however, is not limited to this and liquidcrystals of other types, such as chiral nematic liquid crystals ornematic liquid crystals, may also be used. In addition, materials forthe substrate 14 are not limited to oriented films such as oriented PETfilms, and other film-like sheets can also be used. If a sheet having noaligning action on its surface unlike an oriented PET film is used asthe substrate, it is necessary to form an alignment layer on one surfaceof the sheet that will be brought into contact with the releasingsurface 12A or 22A of the liquid crystal layer 12 or 22.

The first and second embodiments of the invention have been explained bytaking as an example a case where the liquid crystal layer is formed bythe use of liquid crystalline molecules (liquid crystalline monomer oroligomer) polymerizable by the application of ultraviolet light.However, the present invention is not limited to this and is alsoapplicable to a case where the liquid crystal layer is formed by the useof a polymeric liquid crystal. To produce a liquid crystalline transfersheet including a liquid crystal layer made from a polymeric liquidcrystal, a polymeric liquid crystal is firstly applied to the surface ofa substrate having aligning action and is dried to form a liquid crystallayer, and a solvent is sprayed over the adhering surface of this liquidcrystal layer to increase the solvent concentration of the adheringsurface, thereby softening this surface. A liquid crystalline transfersheet in which the surface hardness of the liquid crystal layer is loweron the adhering surface side than on the releasing surface side can thusbe obtained.

EXAMPLES Example 1

A toluene solution (cholesteric liquid crystal solution) containing 33%of a cholesteric liquid crystalline monomer having molecular helicalstructure (cholesteric phase) obtained by mixing a nematic liquidcrystal and a chiral agent was firstly prepared.

To this cholesteric liquid crystal solution was added 5% of aphotopolymerization initiator, for example, Irg 184, Irg 369 or Irg 651(available from Ciba Specialty Chemicals K.K., Japan).

This cholesteric liquid crystal solution was applied to a 50-μm thickoriented PET film, substrate, by using a spinner and was dried at atemperature between normal temperatures (21° C.) and 80° C. to removethe solvent, thereby forming a 10-μm thick uncured cholesteric liquidcrystal layer.

To this cholesteric liquid crystal layer and the substrate, 20 mJ/cm² ofultraviolet light was applied from the cholesteric liquid crystal layerside in an atmosphere of air (oxygen concentration: approximately 20%)at a temperature of 21° C., thereby curing the cholesteric liquidcrystal layer. A liquid crystalline transfer sheet as shown in FIG. 1was thus produced. An extra-high pressure mercury vapor lamp was used asthe light source of the ultraviolet light applied. The above-describedintensity of ultraviolet light is a value obtained by measurement usinga photodetector at 365 nm.

The liquid crystalline transfer sheet thus produced was superposed on aglass plate, receiving object, with the adhering surface of the liquidcrystal layer on the side opposite to the oriented PET film, substrate,facing the surface of the glass plate. This was placed in a laminator toconduct thermocompression bonding under such lamination conditions thatthe roll temperature was 150° C., that the speed was 0.5 m/min and thatthe roll pressure was 0.3 MPa.

Thereafter, the liquid crystalline transfer sheet adhered to the glassplate was cooled to normal temperatures (21° C.) by dissipating theheat, and the oriented PET film, substrate, was peeled. It was easy topeel the oriented PET film, substrate, from the cholesteric liquidcrystal layer; the cholesteric liquid crystal layer was thus securelytransferred to the glass plate. Such a phenomenon was not observed inthe course of separation that the cholesteric liquid crystal layerpartially remained on the oriented PET film or that the cholestericliquid crystal layer was damaged.

Examples 2 & 3

The procedure of Example 1 was repeated, provided that the thickness ofthe cholesteric liquid crystal layer was changed. A liquid crystal linetransfer sheet including a 1-μm thick cholesteric liquid crystal layerwas produced in Example 2, and a liquid crystalline transfer sheetincluding a 5-μm thick cholesteric liquid crystal layer was produced inExample 3.

Each liquid crystalline transfer sheet thus obtained was adhered to aglass plate via thermocompression bonding conducted in the same manneras in Example 1. The cholesteric liquid crystal layer of each liquidcrystalline transfer sheet was successfully transferred to the glassplate.

Example 4

The same cholesteric liquid crystal solution as that prepared in Example1 was applied to the surface of the same oriented PET film as that usedin Example 1 to form an uncured cholesteric liquid crystal layer. Tothis uncured cholesteric liquid crystal layer, 10 mJ/cm² of ultravioletlight was applied from the uncured cholesteric liquid crystal layer sidein an atmosphere of nitrogen (oxygen concentration: 0.5% or less) at atemperature of 21° C., thereby curing the cholesteric liquid crystallayer. A first thin liquid crystal layer was thus formed.

Next, on the surface of this first thin liquid crystal layer cured, asimilar cholesteric liquid crystal layer was directly formed in themanner as described above. To the uncured cholesteric liquid crystallayer thus formed, 0.4 mJ/cm² of ultraviolet light was applied from theuncured cholesteric liquid crystal layer side in an atmosphere ofnitrogen (oxygen concentration: 0.5% or less) at a temperature of 21° C.to cure this layer, thereby obtaining a second thin liquid crystal layercured. A liquid crystalline transfer sheet including a cholestericliquid crystal layer consisting of a laminate of the first and secondthin liquid crystal layers cured was thus finally obtained.

In the above process, although the atmosphere in which ultraviolet lightwas applied to obtain the first thin liquid crystal layer was the sameas that in which ultraviolet light was applied to obtain the second thinliquid crystal layer, the amount of ultraviolet light to be applied toobtain the first thin liquid crystal layer was made more than 20 timesgreater than that of ultraviolet light to be applied to obtain thesecond thin liquid crystal layer.

The liquid crystalline transfer sheet thus produced was adhered to aglass plate via thermocompression bonding conducted under the samelamination conditions as those described before. After cooling thislaminate, the oriented PET film was peeled. The cholesteric liquidcrystal layer was successively transferred to the glass plate. Such aphenomenon was not observed in the course of separation that the firstthin liquid crystal layer partly remained on the oriented PET film orthat the first or second thin liquid crystal layer was damaged.

Example 5

A cholesteric liquid crystal solution was applied to a glass plate,receiving object, and was cured to form a cholesteric liquid crystalfilm. Each one of the liquid crystalline transfer sheets produced inExamples 1 to 4 was superposed on the above glass plate coated with thecholesteric liquid crystal film, with the cholesteric liquid crystallayer of the liquid crystalline transfer sheet facing the cholestericliquid crystal film on the glass plate. This was placed in a laminatorto conduct thermocompression bonding under such lamination conditionsthat the roll temperature was 150° C., that the speed was 0.5 m/min andthat the roll pressure was 0.3 MPa.

Thereafter, each laminate thus obtained was cooled to normaltemperatures (21° C.) by dissipating the heat; and the oriented PETfilm, substrate, was peeled. It was easy to peel the oriented PET film,substrate, from the cholesteric liquid crystal layer of each liquidcrystalline transfer sheet; the cholesteric liquid crystal layer itselfwas securely transferred to the glass plate covered with the cholestericliquid crystal film, receiving object.

Comparative Example

In order to compare with the liquid crystalline transfer sheet made inExample 1, a comparative liquid crystalline transfer sheet was made inthe following manner: the same cholesteric liquid crystal solution asthat prepared in Example 1 was applied to the surface of the sameoriented PET film as that used in Example 1 to form an uncuredcholesteric liquid crystal layer, and 10 mJ/cm² of ultraviolet light wasapplied in an atmosphere of nitrogen (oxygen concentration: 0.5% orless) at a temperature of 21° C. to cure the cholesteric liquid crystallayer.

The liquid crystalline transfer sheet thus produced was adhered to aglass plate under the same lamination conditions as in Example 1, but itwas impossible to transfer the liquid crystal layer to the glass plate.

With respect to the liquid crystalline transfer sheet of Example 1 andthe comparative liquid crystalline transfer sheet, the peel strengthbetween the substrate (oriented PET film) and the cholesteric liquidcrystal layer and that between the cholesteric liquid crystal layer andthe glass plate were measured. The results are shown in Table 1.

TABLE 1 Cholesteric Li- Peel Strength 10⁻³ N/mm quid CrystallineSubstrate-Cholesteric Cholesteric Liquid Transfer Sheet Liquid CrystalLayer Crystal Layer-Glass Plate Example 1 2 12 Comp. Ex. 2  2

As is clear from the data shown in Table 1, in the case of thecomparative liquid crystalline transfer sheet adhered to the glassplate, the peel strength between the substrate and the cholestericliquid crystal layer is equal to that between the cholesteric liquidcrystal layer and the glass plate. In the case of the liquid crystallinetransfer sheet of Example 1 adhered to the glass plate, on the otherhand, there is a great difference between the two peel strengths, andthe peel strength between the cholesteric liquid crystal layer and theglass plate is stronger. This shows that the cholesteric liquid crystallayer was easily transferred to the glass plate without causingunfavorable separation from the glass plate.

Further, with respect to the liquid crystalline transfer sheet ofExample 1 and the comparative liquid crystalline transfer sheet, therate of conversion (degree of polymerization) of the liquid crystallinemolecules in the vicinity of the releasing surface of the liquid crystallayer and that of the liquid crystalline molecules in the vicinity ofthe adhering surface of the liquid crystal layer were determined; andthe results obtained are shown in Table 2. (It is noted that the formerrate of conversion and the latter rate of conversion are proportional tothe degree of cure of the liquid crystalline molecules in the vicinityof the releasing surface of the liquid crystal layer and that of theliquid crystalline molecules in the vicinity of the adhering surface ofthe liquid crystal layer, respectively.)

TABLE 2 Cholesteric Liquid Example 1 Comparative Example CrystallineTransfer Transfer Substrate Transfer Substrate Sheet Side Side Side SideRate of Conversion 1.6 2.7 2.6 2.6 of Cholesteric Liquid CrystallineMolecules

In this measurement, the rate of conversion of C═C bonds present in theregion extending from the surface of each cholesteric liquid crystallayer to the depth of 2–3 μm was confirmed by using IR (infrared)spectra.

The cholesteric liquid crystalline molecules have acrylic groups, andapplication of ultraviolet light causes cleavage of C═C bonds; thereaction thus proceeds and the molecules are cured.

The data shown in Table 2 demonstrate the following: in the comparativeliquid crystalline transfer sheet, the rate of conversion (i.e., degreeof cure) of the liquid crystalline molecules on the releasing surfaceside is nearly equal to that of the liquid crystalline molecules on theadhering surface side; while in the liquid crystalline transfer sheet ofExample 1, the rate of conversion of the liquid crystalline molecules onthe adhering surface side is 59% of that of the liquid crystallinemolecules on the releasing surface side.

With respect to the liquid crystalline transfer sheet of Example 1 andthe comparative liquid crystalline transfer sheet, the state of thesurface (adhering surface) with which the liquid crystalline transfersheet was adhered to the receiving object was analyzed by the use of apendulum impact tester for rigid body (manufactured by A & D Company).The results are respectively shown in FIGS. 6 and 7.

FIGS. 6 and 7 show the relationship between logarithmic decrement, oneindex of the surface hardness of a test sample, and temperature, wherewhen the logarithmic decrement is greater, the surface is softer and hasbetter adhesive properties.

It can be understood from FIGS. 6 and 7 that the liquid crystallinetransfer sheet of Example 1 has strong adhesion to the receiving objectand also a high rate of residual double bonds of the liquid crystallinemolecules as compared with the comparative liquid crystalline transfersheet.

Further, from comparison between FIG. 6 and FIG. 7, it can also beunderstood that the logarithmic decrement of the liquid crystallinetransfer sheet of Example 1 is significantly great on the lowtemperature side. This means that a fluid component is present, even atlow temperatures, on the surface of the cholesteric liquid crystal layerof the liquid crystalline transfer sheet of Example 1. Such acholesteric liquid crystal layer shows fluidity over a wide range oftemperature, so that the distortion of the liquid crystal layer tends tobe eliminated in the course of transfer of the liquid crystal layer to areceiving object. The adhesion between the liquid crystal layer and thereceiving object is thus improved. It is noted that the fluid componentherein means uncured cholesteric liquid crystalline molecules and refersto a monomer or an oligomer having a relatively low molecular weight.

1. A liquid crystalline transfer sheet comprising: a substrate; and aliquid crystal layer formed on a surface of the substrate, the liquidcrystal layer having an adhering surface on a side of the liquid crystallayer opposite from the substrate, the adhering surface being capable ofadhering to a receiving object, and a releasing surface on a substrateside-of the liquid crystal layer, the releasing surface being separablefrom the substrate; wherein: a surface hardness of the liquid crystallayer is lower on the adhering surface side than on the releasingsurface side; and the liquid crystal layer is made from polymerizableliquid crystalline molecules, and a rate of residual double bonds of theliquid crystalline molecules in a vicinity of the adhering surface ofthe liquid crystal layer is higher than that of the liquid crystallinemolecules in a vicinity of the releasing surface of the liquid crystallayer.
 2. The liquid crystalline transfer sheet according to claim 1,wherein the rate of residual double bonds of the liquid crystallinemolecules in the vicinity of the releasing surface of the liquid crystallayer is 60% or less of that of the liquid crystalline molecules in thevicinity of the adhering surface of the liquid crystal layer.
 3. Theliquid crystalline transfer sheet according to claim 1, wherein theliquid crystal layer is composed of a plurality of thin liquid crystallayers successively laminated.
 4. The liquid crystalline transfer sheetaccording to claim 3, wherein each thin liquid crystal layer is madefrom polymerizable liquid crystalline molecules, and a rate of residualdouble bonds of the liquid crystalline molecules in the thin liquidcrystal layer in a vicinity of the adhering surface is higher than thatof the liquid crystalline molecules in the thin liquid crystal layer ina vicinity of the releasing surface.
 5. The liquid crystalline transfersheet according to claim 4, wherein the rate of residual double bonds ofthe liquid crystalline molecules in the thin liquid crystal layer thatforms the releasing surface is 60% or less of that of the liquidcrystalline molecules in the thin liquid crystal layer that forms theadhering surface.
 6. The liquid crystalline transfer sheet according toclaim 1, wherein the liquid crystal layer made from the liquidcrystalline molecules is cholesteric.
 7. The liquid crystalline transfersheet according to claim 1, wherein the substrate is an oriented film.8. The liquid crystalline transfer sheet according to claim 1, whereinan alignment layer is formed on one surface of the substrate, thesurface being brought into contact with the releasing surface of theliquid crystal layer.
 9. A process of producing a liquid crystallinetransfer sheet comprising a substrate and a liquid crystal layer, theprocess comprising: forming the liquid crystal layer on the substrateusing liquid crystalline molecules polymerizable by application ofradiation; and curing the liquid crystal layer by application ofradiation in an atmosphere of air so that a surface hardness of theliquid crystal layer is lower on an adhering surface on a side of theliquid crystal layer opposite from the substrate than on a releasingsurface on a substrate side of the liquid crystal layer; wherein: theadhering surface is capable of adhering to a receiving object; thereleasing surface is separable from the substrate; and a rate ofresidual double bonds of the liquid crystalline molecules in a vicinityof the adhering surface of the liquid crystal layer is higher than thatof the liquid crystalline molecules in a vicinity of the releasingsurface of the liquid crystal layer.
 10. The process of producing aliquid crystalline transfer sheet according to claim 9, wherein theatmosphere of air is an atmosphere with an oxygen concentration of 0.5%or more.
 11. A process of producing a liquid crystalline transfer sheetcomprising a substrate and a liquid crystal layer composed of aplurality of thin liquid crystal layers, the process comprising: forminga first thin liquid crystal layer on a substrate using liquidcrystalline molecules polymerizable by application of radiation; curingthe first thin liquid crystal layer by application of radiation; forminga second thin liquid crystal layer on the cured first thin liquidcrystal layer using liquid crystalline molecules polymerizable byapplication of radiation; curing the second thin liquid crystal layer byapplication of radiation; and forming and curing additional thin liquidcrystal layers as necessary to obtain the plurality of thin liquidcrystal layers; wherein: the liquid crystal layer has an adheringsurface on a side opposite from the substrate, the adhering surfacebeing capable of adhering to a receiving object; the liquid crystallayer has a releasing surface on a substrate side, the releasing surfacebeing separable from the substrate; a degree of cure of each thin liquidcrystal layer is controlled so that a hardness of the liquid crystallayer in a vicinity of the adhering surface is lower than that of theliquid crystal layer in a vicinity of the releasing surface; and a rateof residual double bonds of the liquid crystalline molecules in avicinity of the adhering surface of the liquid crystal layer is higherthan that of the liquid crystalline molecules in a vicinity of thereleasing surface of the liquid crystal layer.
 12. The process ofproducing a liquid crystalline transfer sheet according to claim 11,wherein the degree of cure of each thin liquid crystal layer iscontrolled by adjusting an oxygen concentration of an atmosphere inwhich radiation is applied to the thin liquid crystal layer.
 13. Theprocess of producing a liquid crystalline transfer sheet according toclaim 11, wherein the degree of cure of each thin liquid crystal layeris controlled by adjusting an amount of radiation to be applied to thethin liquid crystal layer.
 14. A process of producing a liquidcrystalline transfer sheet comprising a substrate and a liquid crystallayer composed of a plurality of thin liquid crystal layers, the processcomprising: laminating a desired number of thin liquid crystal layers ona substrate using liquid crystalline molecules polymerizable byapplication of ultraviolet light; and curing the laminated thin liquidcrystal layers by application of ultraviolet light; wherein: the liquidcrystal layer has an adhering surface on a side opposite from thesubstrate, the adhering surface being capable of adhering to a receivingobject; the liquid crystal layer has a releasing surface on a substrateside, the releasing surface being separable from the substrate; a degreeof cure of each thin liquid crystal layer is controlled by adjusting anamount of a photopolymerization initiator to be added to the thin liquidcrystal layer so that a hardness of the liquid crystal layer in avicinity of the adhering surface is lower than that of the liquidcrystal layer in a vicinity of the releasing surface; and a rate ofresidual double bonds of the liquid crystalline molecules in a vicinityof the adhering surface of the liquid crystal layer is higher than thatof the liquid crystalline molecules in a vicinity of the releasingsurface of the liquid crystal layer.